Sweetening process using alkali metal hydroxide containing high-boiling mercaptan salts



Oct. 9, 1951 J. w. RYDER 2,570,278 SWEETEINING PROCESS USING ALKALI METAL HYDROXIDE CONTAINING HIGH BOILING MERCAPTAN SALTS Filed March 29, 1949 JSEDAQATION ZONE I To (55 t w HO F T -L-ATlON 55 ZONE s 5 59; 4 1 J D i i 5 m4 9 fizz? P I O Uh Q) 7 A I? I 5 40 T l THERMAO 4 HEATNG gfi H -11 CAUSTIC TREATING Ii?) 'ZONE 1 5O i'2- I :7: j C2Ac|LED i5 L lGCQ GAsQLINE AS II- T SEC. l II AS;$?Z Z5 [:i9 51 ",m,

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. so 7O F (27 L (INITIAL. CAUfiTIC. l TREATING i7 52 25 a a H h \CONFCZZENTRATION h l ONE 55- 51 I H-Z9 55 TERTIARY CAus'rlc l TQEAT\NG ZONE James Ryder SnQenLor Patented Oct. 9, 1951 SWEETENING PROCESS USING ALKALI METAL HYDR-QXIDE C ON TAINING HIGH-BOILING MERCAPTAN SALTS James W. Ryder, Watchung, N. J., assignor to Standard Oil Development Company, a, corporation of Delaware Application March 29, 1949, Serial No. 84,216

Claims.

The present invention is concerned with an improved sweetening process and is more particularly concerned with the removal of mercaptan compounds from oils containing the same. The invention is especially directed toward the removal of those mercaptan compounds which have heretofore been relatively difficult to remove from hydrocarbon fractions especially from those hydrocarbon fractions which boil in the motor fuel, kerosene, diesel oil and heating oil, oiling ranges. The invention is particularly coi'icerned with the removal of mercaptan compounds from a petroleum oil boiling in the motor fuel boiling range. This is accomplished by treating the oil boiling in the motor fuel boiling range with a caustic solution which has been used to treat an oil boiling above the motor fuel boiling range, as for example, in the heating oil boiling range. The invention is particularly adapted for the removal of mercaptan compounds from prime cut naphthas by utilizing as a treating solution for the prime cut naphthas a caustic solution which has been utilized in the treatment of cracked naphthas and in the treatment of petroleum fractions boiling in the range above the motor fuel boiling range.

It is well known in the art to treat petroleum oils by various procedures in order to remove objectionable constituents as for example sulfur compounds therefrom. For example, it is known to treat petroleum fractions boiling in the motor fuel boiling range and in the general range below about 760 F. with various chemical reagents such as sodium or potassium hydroxide solutions. Also, it has long been known that weakly acidic materials, such as mercaptans, are reactive to various degrees with these basic treating agents. One method utilized for the removal of mercaptans from hydrocarbon streams has been to treat the streams with a reagent which is insoluble in the hydrocarbon stream but which contains alkali type of material which reacts with mercaptans. Such treating reagents have normally been aqueous caustic solutions. The reaction product is a basic salt of the mercaptan. Normally such salts exhibit some solubility in the aqueous phase, this solubility decreasing as the molecular weight or branchiness of the hydrocarbon structure of the mercaptan increases. The art has generally recognized in the treating of hydrocarbon streams with aqueous solutions such as strong or weak caustic in a continuous type process, that the equilibrium is generally unfavorable for mercaptan extraction. Therefore, caustic solutions when they are only spent to a small extent in mercaptide salts will not give any further reduction in mercaptan content despite the' fact that these treating solutions have a large residual free caustic content. The art has also recognized that the efficiency of the spent caustic can be partially or fully restored by regeneration of these solutions. The regeneration normally consists of removal of a part or all of the mercaptide salts by means of hydrolysis and vaporization or by oxidation.

One method is to employ steam regeneration for the revivification of caustic solution spent in mercaptan removal from hydrocarbon streams. The steam regeneration is normally carried out in a packed tower or a tower provided with bubble plates or pierced type tray plate design. The steam hydrolizes the mercaptide salts to the corresponding mercaptans which are stripped out by the steam vapors and taken overhead. The regenerator also serves as a means of adjusting the gravity of the caustic solution which has been shown to be critical. The regenerated caustic is withdrawn from the bottom of the regeneration tower and reused in the extraction stage of the system wherein the caustic contacts the hydrocarbon stream through suitable mixing devices for further extraction of mercaptans by the same caustic solution.

It has also been found that certain constituents will function as solubility enhancing agents. These constituents tend to improve the partition coefficient of the acid mercaptan in the aqueous phase and to increase the solubility of the mercaptide salts in the aqueous phase, particularly when the mercaptan is of a relatively high molecular weight. As mentioned, mercaptans are difilcult to extract from hydrocarbon solution by straight aqueous alkali solutions. This is A particularly true of the higher molecular weight mercaptans. In general, this is occasioned by the fact that the two phases are not miscible and that good contact must be obtained to allow the alkali metal ion to react with the mercaptan hydrocarbon. Furthermore, once the reaction has occurred, it is essential that if the extraction of the mercaptan is to be accomplished the reaction product must be soluble in the aqueous layer. As pointed out heretofore, the higher the molecular weight of the mercaptan the lower the solubility of the corresponding mercaptide salt in the aqueous phase. The solubility enhancing agents tend to increase this solubility and thus increase the effectiveness of the alkali treating solution.

The exact chemical nature of many of these solubility enhancing agents is not entirely known. However, it is felt that they comprise the lower molecular weight aliphatic acids and the various substituted phenolic compounds. It has been found that phenolic type materials having alkyl side chains containing less than about 3 carbon atoms are not effective by themselves as solubility promoters and that organic carboxylic acids having less than about 3 carbon atoms per molecule are likewise not very effective. However, as the length of the side chain of the phenols increases or the length of the aliphatic acid hydrocarbon chain increases, their effectiveness as solubility promoters increases greatly. However, the solubility of the acids or phenols themselves in the caustic solution decreases with increasing hydrocarbon chain length and thus limits the molecular weight of acid or phenolic type material which can be employed.

It has now been discovered that providing a particular arrangement and sequence of treating stages be employed, unexpected desirable results are secured. As pointed out, it has been discovered that if a relatively low boiling hydrocarbon fraction is contacted with at least a part of a caustic solution which has been previously used to treat a relatively higher boiling fraction unexpected and desirable results are secured. The invention is particularly adapted for the sweetening of those hydrocarbon fractions boiling in the motor fuel boiling range, as for example, those fractions boiling in the general range from about 80 to 420 F. If fractions boiling in this range are contacted with a caustic solution which has been utilized to remove mercaptan compounds from higher boiling fractions as for example those fractions boiling in the range from about 400 F. to 700 F. unexpected desirable results are secured.

tration is assumed to be a petroleum crude oil, is

introduced into distillation zone if! by means of line i. Temperature and pressure conditions in zone I are adapted to segregate the oil into various hydrocarbon fractions having different boiling ranges. Zone It may comprise any suitable number and arrangement of distillation or equivalent stages. A fraction boiling in the motor fuel boiling range (80 I l-420 F.) is removed overhead from zone It by means of line 2. This fraction is introduced into separation zone wherein normally gaseous constituents are segregated from the normally liquid constituents. A stream comprising normally gaseous constituents .is removed overhead from zone 20 by means of line 3 while a condensed hydrocarbon stream comprising normally liquid constituents is removed from the bottom of zone 2i] by means of line '4. This stream comprises a prime cut (uncracked) naphtha fraction boiling in the general range from about 80 to 420 F. It is to be understood that this stream may comprise a light naphtha stream or any other boiling range fraction which it is desired to segregate under any particular operating conditions. In many operations it may be desirable to segregate a light naphtha and a heavy naphtha prime cut fraction. The heavy naphtha prime cut fraction generally boils in the range from about 200 to 420 F. while the light naphtha prime cut fraction has a general boiling range from about 80 F. to 250 F. A gas oil or heating oil fraction is segreture and pressure conditions are adjusted to produce lower boiling hydrocarbon constituents. The cracked distillate stream is removed from zone 30 by means of line 8 and introduced into a distillation zone it. Temperature and pressure conditions are adjusted in zone 36 to segregate the various boiling cracked fractions. Uncondensed hydrocarbon constituents are removed overhead from zone 30 by means of line 9 while a cracked fraction boiling in the motor fuel boiling range is removed from zone id by means of line ll. A cracked gas oil or heating oil fraction is removed from'zcne 36 by means of line l2 and introduced into a treating zone wherein the same is contacted with an aqueous caustic solution. For the purpose of illustration it is assumed that the caustic solution comprises a sodium hydroxide solution which is introduced in-- to zone by means of line 53 and withdrawn by means of line M. It is to be understood that treating zone 56 may comprise any suitable number and arrangement of mixing and treating stages.

The treated heating oil is withdrawn from zone 59 by means of line l5 and handled or further refined as desired. If the heating oil contains-mercaptans to any appreciable degree and if it is desired to remove these constituents they are generally removed by treating the heating oil fraction with a lead sulphide or equivalent reagent.

The caustic solution utilized in treating the heat-.

ing oil fraction does not react to any extent with the mercaptans present but only with the complex acids and phenolic constituents present. The exact chemical nature or structure of these constituents is not known.

The spent caustic solution removed by means of line it is at least partially utilized to treat lower boiling hydrocarbon fractions in a manner as hereinafter described. The cracked naphtha withdrawn by means of line i l is introduced into a distillation and stabilization zone as wherein hydrogen sulphide and normally gaseous hydrocarbons are removed therefrom. These constituents are removed overhead from zone 60 by means of line it. A stabilized cracked naphtha is withdrawn from zone Bil by means of line H and introduced into an initial caustic treating zone 10 wherein the same is contacted with at least a portion of the spent caustic solution withdrawn from zone 50 which is introduced into zone ill by means of line l4. Fresh caustic may be introduced into zone l'll by means of line 19. The spent caustic solution withdrawn from the initial caustic treating zone may be recycled in part by means of line 5 I.

The spent caustic solution is withdrawn from zone l9 by means of line 2! and introduced into a concentration zone 0. Zone is operated under conditions to remove overhead by means of line 22 water and to remove as a bottoms by means of line 23 a concentrated caustic solution.

In accordance with a preferred adaptation of bodiments of the same.

the invention the treated cracked naphtha is withdrawn from zone by means of line 24 and introduced into a secondary caustic treating zone 90. In zone 90 the oil is contacted with a relagated by means of line 33 and used to contact the prime cut naphtha segregated by means of line Example An uncracked hydrocarbon fraction secured from a West Texas crude and boiling in the range from about 80 F. to 420 F. had a copper tively concentrated caustic solution which is in- 5 nu mber of 56. Portions of this naphtha were ggggg g 2 it g i g g zfi zifi gi treated successively with treats of a sodium Withdrawn from ztme 9a g be recycled in part hydroxide solution. The treating solution conby means of line 52 The treated oil is withdrawn tamed of acld and 120 grams per liter of from zone 90 by means of line 2'! and introduced 10 g gf jgg i g g igzigg' i f gfig g i fs ggfi starts: Ass-area? g; q g an re easing e acid oi y aci ying. The reg; tra lgich rfiri'sfggfilfit iin is 1 5g; ?g% xg sults are shown the table under operation 1. In operation 2 he same West Texas naph ha lme and ltllldawn yg fi g 2 2 3 15 was treated except that the caustic solution cona Ion o b p hed b tained 90 grams per liter of free sodium hydroxide span i. 1c :3 u Ion may 8 accomp 18 y and 40% of an acid oil which was secured by ii gg ig on free of mercaptan treating a cracked gas oil boiling in the range from means of 20 are resistant assassinate: line 3| and handled or further refined as desired. under operation 2 g fisg z i g g g gg i g g z gg In operations 3, 4 and 5 the caustic solutions the caustic treating zones 50 10 90 and I00 may emPloyed in 9 1 and 2 were utilized in comprise any Suitable number and arrangement various proportions in accordance with the presof treating stages. It is also to be understood em process that circulation or partial recirculation of the In olsferfdtlons 6, 7 and 3 acidltlonal runs were spent caustic Solution may be carried t made similar to those made with respect to oper- A portion of t concentrated caustic S0111- ations 1 and 5 except that the naphtha treated tion withdrawn from zone so is preferably segrewas a cracked naphtha boiling i the rang from 80 F. to 420 F. and which was secured from a Quiriquire crude.

Copper number reduction of West Texas virgin naphtha, originally 56 Cu. No., employing 15 volume per cent batch treats of extracting solution Extracting Solution, Composition G On No. 1 or Doctor Test 2 After Treat No.

l rav- O v 1 i 1 perao l 2 3 4 5 F tion Descnptlon Per Cent l Aliphatic Acid Extract from Quiriquire Cracked Naphtha Containing acid oil and 120 100 9 3 1 1 DNP DNP gJl. free NaOH 2 Alkyl Phenol Extract from Quiriquire Cracked Gas Oil Containing 40% Acid Oil and 90 g./l. 100 8 2 1 DNP JDNP Pass free NaOH 7 3 g 7 1 DNP Pass 4 2g s 1 1 DNP Pass 5 25 .2 Above 75 6 Pass Same on Quiriquire cracked naphtha, originally The copper number is the milligrams of mercaptan sulfur present in 100 ml. of sample under test which will react with a standard copper ammonium sulphate solution.

2 The doctor test is a means of making sure that hydrocarbon fractions are free of mcrcaptans which render the fraction sour.

A sample of the hydrocarbon is shaken with a sodium plumbite solution and a trace of free sulfur.

If after shaking, the hydroca"- hon, plumbite solution, and sulfur have not changed color, mercaptans are absent, the stock is sweet and the doctor test is Pass. If discoloration occurs mercaptans are present and the test does not pass (DNP).

4 in zone H0. The spent caustic solution is withdrawn from zone H0 by means of line 34 while the treated oil substantially free of mercaptan is withdrawn from zone H0 by means of line 35. Recirculation of the spent caustic withdrawn from zone H0 may be accomplished by means of line 54.

The present invention may be further understood by the following example illustrating em- From the above it is apparent that unexpected desirable results are secured when employing a caustic solution which has been preferably utilized to treat a higher boiling hydrocarbon.

The present invention is broadly concerned with the removal of mercaptan compounds from relatively low boiling petroleum oils utilizing a caustic solution which has previously been used in the treatment of relatively high boiling petroleum oils. A particular embodiment of the intion for the treatment of petroleum oils boiling below about 420 F.

The low boiling fraction being treated may be a cracked fraction boiling in the motor fuel boiling range. However, a particular adaptation of the present invention is to treat a prime cut naphtha boiling in the motor fuel boiling range in accordance with the present process. If a prime cutfraction be treated, it is very desirable that the caustic solution be one that has been previously used to contact a naphtha boiling in the motor fuel boiling range and which has also been used to contact a hydrocarbon fraction boiling above the motor fuel boiling range.

The invention may be utilized for the removal of mercaptan compounds from any type of feed oil. It is particularly adapted for the removal of mercaptan compounds from petroleum. hydrocarbon fractions boiling below about 700 F., as for example for the removal of mercaptan compounds from hydrocarbon fractions which boil in the motor fuel, kerosene, diesel oil and gas oil boiling ranges. Particularly desirable feed oils are oils which boil in the motor fuel boiling range and which have been derived from naphthenic or aromatic type crudes and which contain.

naphthenic acids or other similar constituents. These naphthenic acids upon cracking apparently formed various related low molecular weight aliphatic acids. A preferred feed oil for the segregation of solubility enhancing agents of the present invention is a feed oil from a naphthem'c crude which has been thermally cracked. The cracking temperatures are above about 750 F. and preferably in the range from about 850 F.

to 950 F. Cracking pressures are above about 200 lbs. per sq. in. and preferably in the range from about 750 to 1000 lbs. per sq. in. gauge.

Thus, it is apparent that if a prime cut naphtha is to be treated, it is very desirable not only that the caustic solution be one that has been previously used to contact a higher boiling hydrocarbon fraction, but also one that has been used to contact a cracked fraction boiling in the motor fuel boiling range. It is within the scope of the present invention to utilize externally added solubility enhancing agents which may be segregated from other feed streams or manufactured in other processes. In general the invention comprises the use of an alkali solution which contains relatively low molecular weight Or relatively low boiling aliphatic type acids in conjunction with higher boiling phenolc type constituents.

A disclosed in the description of the drawing. a three-stage alkali treatment is utilized. With respect to these stages the fresh sodium hydroxsodium hydroxide in the fresh caustic solution should be below about and is preferably in the range from about to 3 70. When the concentration of the caustic solution is specified hundred grams of mixture comprising sodium hydroxide and water. By utilizing a solution of this character it has been found that the most effective solubility enhancing agents can be readily removed from'the oil. Furthermore, it

has been found that when utilizinga-caustic solution having these critical concentrations the various phenolic compounds and otherconstituents' which apparently greatly adversely afiect the efiiciency of the solubility enhancing agents are not picked up-by the caustic solution but remain in the oil stream being treated. Thus,a very sharp selective separation is mad between the desired solubility enhancing agents which are picked up in the caustic solution and the other acidic constituents which adversely affectthe. efiiciency of the solubility enhancing 7 agents. While a once through operation may be in the present invention, it is meant for example,: 3

the grams of sodium hydroxide present in a employed with respect to the caustic stream, it is preferred to employ at least a partial recirculationof the caustic stream withdrawn from the initial zone. The spent caustic stream withdrawn from the initial caustic contacting zone and not recirculated is controlled so asto be spent in the range from about to 100%. A preferred method of operation is to control the various factors so that the spent caustic stream withdrawn from the initial zone and not recirculatedis spent to a degree in the range from about 30% to It is to be understood that the term spent when used in the present invention means that percentage of sodium ion which has reacted with organic carboxylic acids, as for example with aliphatic acids, to produce sodium salts of these acids. In the operation, upon initial contact a portion of the sodium might react with the various phenols to form sodium phenolates. However, upon recirculation the phenolates are reconverted to the phenols and are displaced from the aqueous solution with the resulting formation of sodium salts of aliphatic acids. Thus, the amount of sodium not spent may be available either in the form of the hydroxide or as a sodium phenolate. Although the amount of sodium available may be entirely present as a sodium phenolate, it is preferred that a small amount of the sodium also be available as the hydroxide.

The concentration of the caustic specified in the present invention with respect to the initial stage is the concentration at which effective results are secured. It is obvious that stronger caustic solutions could be initially used, as for example 10 B. or higher and somewhat similar results obtained by recirculating the spent caustic until from about to 100% is spent, that is, until 70% to 100% of the sodium ion is reacted to form sodium salts of aliphatic acids. When employing caustic solutions having greater concentrations than about 7 B., it is preferred that the caustic solution withdrawn from the initial stage be spent in excess of about The strength of the fresh caustic added in the secondary caustic treating stage is preferably in the range from about 24% to' 30% NaOH. Here again although a once through operation may "be utilized, itis preferred to employ at least partial recycling under conditions so that the total caustic stream utilized in thetreatment of the oil in thesecondary stage does not have a concentration less than about 12% to 15% NaOI-I. If the spent caustic stream has approximately these critical minimum concentrations a portion of the stream is withdrawn from the system while a portion of the stream maybe recycled and mixed with fresh caustic solution having a concentration as specified above. By operating in this manner the phenolic and other acidic constituents which adversely affect the efficiency of the solubility enhancing agents in the tertiary stage for the removal of mercaptans are effectively removed from the oil. The amount of caustic solution employed by volume based upon the oil may vary appreciably. However, it is preferred that in the initial and in the secondary stages the amount of caustic solution employed based upon the oil be sufiicient to obtain good mixing and remove desired acidic constituents, which by volume is in the range from about to 50%.

The concentration of the caustic solution employed in the tertiary stage is less than the concentration of the caustic used to contact the oil in the secondary stage. As previousl mentioned the concentration of the caustic solution employed in the secondary stage is preferably in the range from about 12% to 15%. The concentration of the caustic employed in the tertiary stage for any particular operation is less than these specified concentrations for the secondary stage, and is in the general range from about 9% to 12%. It is preferred that the concentration of the caustic employed in the tertiary stage be from about 1% to 3% less than the caustic employed in the secondary stage.

These desired concentrations of the caustic for utilization in the tertiary stage may be secured by controlling the amount of fresh caustic added and the extent to Which the spent caustic is regenerated. Another factor which must be taken into consideration is adjusting the desired strength of caustic in the tertiary stage is the amount of water added wi h the spent caustic containing the solubilit enhancing agents from the initial stage. The amount of caustic solution employed in the tertiary stage is in the range from about 5 to 50% based upon the volume of oil being contacted. The concentration of the solubility enhancing agents present in the caustic solution introduced into the tertiary stage is preferably in the range so that from about 25% to 75% of the total sodium or other alkali metal is present as a sodium salt.

Having described the invention it is claimed:

1. Process for the extraction of mercaptan compounds from hydrocarbon fractions which comprises contacting a hydrocarbon fraction boiling in the range of about 400 to 700 F. with a caustic solution, segregating therefrom a partially spent caustic solution containing mercaptan salts, and thereafter contacting a hydrocarbon fraction boiling in the range of about 80 to 420 F. with said partially spent caustic solution containing mercaptan salts, and withdrawing said caustic solution whereby the mercaptan content of the fraction boiling in the motor fuel range is substantially reduced.

2. Process for the extraction of mercaptan compounds from cracked hydrocarbon fractions which comprises contacting a cracked hydrocarbon fraction boiling in the range of about 400 to 700 F. with a caustic solution, and segregating a partially spent caustic solution containing mercaptan salts, and thereafter contacting a cracked hydrocarbon fraction boiling in the range of about to 420 F. with said partially spent caustic solution containing mercaptan salts, and withdrawing said caustic solution whereby the mercaptan content of the fraction boiling in the motor fuel range is substantially reduced.

3. Improved process for the removal of mercaptan compounds from a prime cut naphtha boiling in the range of about 80 to 420 E. which comprises contacting a cracked hydrocarbon fraction boiling in the range of about 400 to 700 F. with a caustic solution providing a partially spent caustic solution containing mercaptan salts, and thereafter contacting said partially spent caustic solution with said prime cut naphtha, and withdrawing said caustic solution, whereby the mercaptan content of the fraction boiling in the motor fuel range is substantially reduced.

4. Improved process for the production of a. mercaptan free hydrocarbon fraction boiling in the range of about 80 to 420 R, which comprises segregating a prime cut fraction boiling in the range of about 80 to 420 F., and a fraction boiling in the range of about 400 to 700 F., from a crude oil, thermally cracking said higher boiling fraction and segregating a cracked fraction boiling in the range of about 80 to 420 F'., and a cracked fraction boiling in the range of about 400 to 700 F., treating said higher boiling cracked fraction with a caustic solution providing a partially spent caustic solution containing mercaptan salts, segregating said partially spent caustic solution and utilizing at least a portion thereof to contact said lower boiling cracked fraction and withdrawing said caustic solution, whereby the mercaptan content of the said lower boiling fraction is materiallv reduced.

5. Process as defined by claim 4 wherein the spent caustic withdrawn after treatment of the said lower boiling cracked fraction is used to treat said prime cut fraction boiling in the range of about 80 to 420 F., whereby the mercaptan content of said prime cut fraction is materially reduced.

JAMES W. RYDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,258,279 Caselli et a1. Oct. 7, 1941 2,324,927 Heilman July 20, 1943 2,347,515 Shmidl Apr. 25, 1944 2,394,662 Camp et al Feb. 12, 1946 2,457,975 Bolt Jan. 4, 1949 

1. PROCESS FOR THE EXTRACTION OF MERCAPTAN COMPOUNDS FROM HYDROCARBON FRACTIONS WHICH COMPRISES CONTACTING A HYDROCARBON FRACTION BOILING IN THE RANGE OF ABOUT 400* TO 700* F. WITH A CAUSTIC SOLUTION, SEGREGATION THEREFROM A PARTIALLY SPENT A CAUSTIC SOLUTION CONTAINING MERCAPTAN SALTS, AND THEREAFTER CONTACTING A HYDROCARBON FRACTION BOILING IN THE RANGE OF ABOUT 80* TO 420* F. WITH SAID PARTIALLY SPENT CAUSTIC SOLUTION CONTAINING MERCAPTAN SALTS, AND WITHDRAWING SAID CAUSTIC SOLUTION WHEREBY THE MERCAPTAN CONTENT OF THE FRACTION BOILING IN THE MOTOR FUEL RANGE IS SUBSTANTIALLY REDUCED. 